The evolution of nuclear reactor systems of the water reactor type has led to the development of such systems in the 200 C MWe plus range and to utilization of stainless steel reactor pressure vessels wherein operating pressure is about 1000 psi. These reactor vessels have reached an internal diameter of about 13 m. Manufacture of vessels of this size involves special fabrication requirements not easily met, especially since fabricators capable of fulfilling such a manufacturing task are few. Also with a practical steel vessel diameter limitation of about 13 m, finding available surface space on a vessel to effect penetration of the many devices and appurtenant components which must be fixed on the vessel, including control rods, piping, instrumentation and cooling lines, is a problem. Because of the many penetrations in the head and wall of the pressure vessel, the latter must be placed within a containment structure for isolating leaked fluids from the reactor environs. The result is an undesirably complex reactor pressure vessel structure and less than optimal system capacity.
To address this problem, a prestressed concrete reactor pressure vessel comprising a steel inner liner, an intermediate insulative layer (e.g., concrete) and an outer prestressed concrete encasement was proposed in U.S. Pat. No. 5,204,054. In accordance with the design disclosed in that patent, a prestressed concrete cover sits on top of the prestressed concrete outer vessel. The steel inner liner passes up through a central opening in the cover and itself is closed by a convex dished head made of steel. Cooling passages are formed at the interface of the outer vessel part with the insulative layer and at the interface of the steel inner liner with the insulative layer.
The use of a prestressed concrete reactor pressure vessel allows for a very large fuel core, simplifies the approach to plant safety and provides a mild response to a plant transient. The use of prestressed concrete construction removes the current size limitations imposed by the steel reactor vessels due to the steel fabrication capacity. This enables the design of a super large-sized natural circulation reactor having a low-power-density core and high plant output power.